U.S. Pat. No. 4,976,207 discloses apparatus for carrying out a combustion process including a combustion chamber and a secondary or post-combustion chamber. Air is injected into the post-combustion chamber. If desired, the air may be preheated. Sensors are provided within the combustion chamber, the post-combustion chamber and an air supply conduit so as to control operation of a device for preheating the air in response to detected sensed signals from the sensors.
U.S. Pat. No. 4,279,208 relates to method and apparatus for the treatment of industrial wastes. At least some of the constituent materials of the waste are subjected to combustion or pyrolysis to produce flue gases. The supply of an oxygenated combustion-supporting gas to the waste is regulated as a function of at least one predetermined characteristic of the flue gas. For example, the oxygenated gas may be a mixture of air and substantially pure oxygen, and the feed rate of one or both of these gases may be regulated in response to either the sensed temperature or the oxygen content of the flue gases.
The control systems disclosed in U.S. Pat. Nos. 4,976,207 and 4,279,208 are essentially reactive in nature. They suffer from the disadvantage that if the waste material has a randomly varying calorific value, occasions can arise when there is a temporary deficiency of oxygen in the secondary combustion region. As a result the flue gases can contain, for a period of time, an unacceptably high concentration of carbon monoxide or residual particles of carbonaceous material. Moreover, by the time the reactive control system has adjusted the oxygen concentration (in the case of the apparatus disclosed in U.S. Pat. No. 4,279,208) or the temperature of the air (in the case of the apparatus shown in U.S. Pat. No. 4,976,207) the demand for oxygen may have subsided. As a result, such control systems may be wasteful and yet fail to produce an adequate quality flue gas.